The present invention relates to particulate, expandable styrene polymers containing graphite particles, their production and foams produced therefrom.
Expanded polystyrene foams have been known for a long time and have proven themselves in many fields. Such foams are produced by foaming polystyrene particles impregnated with blowing agents and subsequently welding together the foam particles produced in this way to give moldings. A significant application area is thermal insulation in building and construction.
In many applications of foams, in particular in building and construction, it is a requirement that the foams be self-extinguishing. Although it is known that this can be achieved by addition of flame retardants, e.g. bromine compounds, whether a foam passes a particular burning test depends on various factors such as composition and density of the foam, type and amount of flame retardant and also type and amount of further additives.
The foam boards made of expanded polystyrene foam which are used for thermal insulation usually have densities of at least 30 g/l, since the thermal conductivity of the expanded polystyrene foam has a minimum at these densities. To save material, it would be desirable to use foam boards having lower densities, in particular xe2x89xa615 g/l, for thermal insulation. The production of such foams is not a problem in technical terms. However, such foam boards having a lower density have a drastically worse thermal insulation performance, so that they do not meet the requirements for thermal conductivity class 035 (DIN 18 164, Part 1).
It is known that the thermal conductivity of foams can be reduced by incorporation of athermanous materials such as carbon black, metal oxides, metal powder or pigments.
Thus, EP-A 372 343 describes polystyrene foams containing from 1 to 25% by weight of carbon black. The carbon black has a particle size of from 10 to 100 nm and a surface area of from 10 to 1500 m2/g. The polystyrene foams described there are predominantly produced by the extrusion method and preferably have a density of 32-40 g/l, as is typical for these foams. The addition of flame retardants is mentioned, but the expanded polystyrene foams described in the examples containing 1.7% by weight of hexabromocyclododecane do not pass the burning test B2 in accordance with DIN 4102.
WO 94/13721 describes similar foams in which the size of the carbon black particles is  greater than 150 nm.
EP-A 620 246 describes moldings made of expanded polystyrene foam which contain a particulate athermanous material, in particular carbon black, and also graphite. The density of the moldings is less than 20 g/l. The particles are preferably incorporated into the moldings by surface coating of the prefoamed polystyrene beads or by embedding into the not yet foamed polystyrene granules. However, this distribution of the particles on the surface leads to severe impairment of the welding together of the prefoamed beads and consequently to foams of low quality; in addition, the particles can be rubbed off from the surface of the molding. In both cases, the particles are in any case not homogeneously distributed in the interior of the polystyrene particles; an addition of flame retardants is not described.
A similar process is described in GB-A 1 588 314, according to which antistatic polystyrene foams are produced by coating unfoamed or prefoamed particles with a graphite suspension.
It is an object of the present invention to provide expandable styrene polymers containing graphite particles which can be processed to give expanded polystyrene foams which have both a low density and a low thermal conductivity and have good processing properties, good physical properties and, in particular, very good flame retardant properties.
We have found that this object is achieved by particulate, expandable styrene polymers containing homogeneously distributed graphite particles and can be processed to give foams which have a density of  less than 35 g/l and are preferably self-extinguishing and pass the burning test B2 (in accordance with DIN 4102).
The invention also provides processes for producing the expandable styrene polymers and also provides the expanded polystyrene foams produced therefrom.
For the purposes of the present invention, expandable styrene polymers are styrene polymers containing blowing agents.
The polymer matrix present in the expandable styrene polymers of the present invention is, in particular, homopolystyrene or a styrene copolymer containing up to 20%, based on the weight of the polymers, of ethylenically unsaturated comonomers, in particular alkylstyrene, divinylbenzene, acrylonitrile or xcex1-methylstyrene. Blends of polystyrene and other polymers, in particular with rubber and polyphenylene ether are also possible.
The styrene polymers can contain the customary and known auxiliaries and additives, for example flame retardants, nucleating agents, UV stabilizers, chain transferrers, blowing agents, plasticizers, pigments and antioxidants.
The expandable particles are coated with the customary and known coating materials, for example metal stearates, glyceryl esters and finely divided silicates.
The particle size is preferably in the range 0.2-2 mm.
The graphite used preferably has a mean particle size of from 1 to 50 xcexcm, in particular from 2.5 to 12 xcexcm, a bulk density of from 100 to 500 g/l and a specific surface area of from 5 to 20 m2/g. Either natural graphite or milled synthetic graphite can be used. The graphite particles are preferably present in the styrene polymer in amounts of from 0.05 to 25% by weight, in particular from 2 to 8% by weight. Surprisingly, it has been found that graphite particles are effective even in amounts of less than 0.5% by weight.
A problem associated with the use of graphite particles is the ready flammability of the expanded polystyrene foams containing graphite particles. Thus, graphite-containing polystyrene foams have hitherto not been able to pass the burning tests required for use in building and construction (B1 and B2 in accordance with DIN 4102).
To rectify this defect, flame retardants, particularly ones based on organic bromine compounds, are added to the expandable styrene polymers in a preferred embodiment of the invention. The bromine compound (without a synergist) should be added in an amount of more than 3% by weight, based on the weight of the expandable styrene polymers. B1 and B2 are not passed when the customary amount of flame retardant is used. The organic bromine compounds should have a bromine content of xe2x89xa770% by weight.
Surprisingly, this amount of flame retardants leads to no deterioration whatever in the mechanical properties of the expanded polystyrene foams containing carbon black.
Particularly suitable flame retardants are aliphatic, cycloaliphatic and aromatic bromine compounds, for example hexabromocyclododecane, pentabromomonochlorocyclohexane and pentabromophenyl allyl ether.
The effect of the bromine-containing flame retardants is considerably improved by addition of Cxe2x80x94C- or Oxe2x80x94O-labile organic compounds. Examples of suitable flame retardant synergists are bicumyl and dicumyl peroxide. A preferred combination comprises 0.6 to 5% by weight of an organic bromine compound and 0.1 to 1.0% by weight of the Cxe2x80x94C- or Oxe2x80x94O-labile organic compound.
The expandable styrene polymers of the present invention can be produced by various methods.
In a preferred embodiment, the graphite particles are mixed with a melt of the styrene polymer, preferably in an extruder. At the same time, the blowing agent is metered into the melt. The graphite particles can also be compounded into a melt of styrene polymer containing blowing agent; in this case, it is convenient to use oversize and undersize fractions of polystyrene beads containing blowing agent formed in a suspension polymerization. The polystyrene melt containing blowing agents and graphite particles is extruded and granulated to form granules containing blowing agent. Since graphite has a strong nucleating action, the compounded polystyrene should be quickly cooled under pressure after extrusion in order to avoid foaming. For this reason, an underwater granulation under pressure is advantageously carried out.
It is also possible to add the blowing agent to styrene polymers containing graphite particles in a separate process step. Here, the granules are then impregnated with the blowing agent, preferably in aqueous suspension.
In all three cases, the finely divided graphite particles can be added directly to the polystyrene melt. The graphite particles can also be added in the form of a concentrate in polystyrene. However, preference is given to introducing polystyrene granules and graphite particles together into an extruder, melting the polystyrene and mixing it with the graphite.
It is in principle also possible to incorporate the graphite particles during the course of the suspension polymerization. Here, they can be added prior to suspending the monomeric styrene or added to the reaction mixture during the course of the polymerization, preferably during the first half of the polymerization cycle. The blowing agent is preferably added during the course of the polymerization, but it can also be incorporated into the styrene polymer afterwards. Here, it has been found that it is favorable in terms of the stability of the suspension for a solution of polystyrene (or an appropriate styrene copolymer) in styrene (or the mixture of styrene with comonomers) to be present at the beginning of the suspension polymerization. Preference is given to starting from a 0.5-30% strength by weight, in particular from 5 to 20% strength by weight, solution of polystyrene in styrene. This can be achieved by dissolving fresh polystyrene in monomers, but use is advantageously made of oversize and undersize fractions which have been sieved out in the fractionation of the variously sized beads obtained in the preparation of expandable polystyrene. In practice, such otherwise unusable oversize and undersize fractions have diameters of greater than 2.0 mm or less than 0.2 mm. Recycled polystyrene and recycled polystyrene foam can also be used. Another possibility is to prepolymerize styrene in bulk up to a conversion of from 0.5 to 70% and to suspend the prepolymer together with the graphite particles in the aqueous phase and complete the polymerization.
The blowing agent is added in the customary amounts of about 3-10% by weight, based on the weight of the polymer. As blowing agents, use is usually made of aliphatic hydrocarbons having from 3 to 10, preferably from 4 to 6, carbon atoms.
The novel expandable styrene polymers containing carbon black can be processed to produce polystyrene foams having densities of 5-35 g/l, preferably from 8 to 25 g/l and in particular 10-15 g/l.
For this purpose, the expandable particles are prefoamed. This is usually achieved by heating the particles by means of steam in prefoamers.
The particles which have been prefoamed in this way are then welded together to give moldings. For this purpose, the prefoamed particles are introduced into molds which do not close in a gastight manner and are treated with steam. After cooling, the moldings can be taken from the mold.
A further surprising effect of the addition of graphite particles is that it can reduce the cooling time until welded foam blocks can be removed from the mold. Thus, for example, an addition of from 0.5 to 5% by weight of graphite leads to a shortening of from 10 to 90% in the cooling time.
The foams produced from the expandable styrene polymers of the present invention have an excellent thermal insulation capacity. This effect is particularly distinct at low densities. Thus, addition of 2% by weight of graphite to an expandable styrene polymer enables the thermal conductivity at a foam density of 10 g/l to be reduced from 44 mW/mxc2x7K to less than 35 mW/mxc2x7K.
The present invention further provides expanded polystyrene foams which have a density of  less than 35 g/l and contain from 0.05 to 25% by weight of homogeneously distributed graphite particles and whose thermal conductivity is reduced sufficiently for the foams to meet the requirements of thermal conductivity class 035 (in accordance with DIN 18 164, Part 1, Table 4) and are preferably self-extinguishing and pass the burning test B2 (in accordance with DIN 4102).
The ability to reduce the density of the styrene polymers significantly at the same thermal conductivity allows material savings to be realized. Since, compared to conventional expandable styrene polymers, the same thermal insulation performance can be achieved at significantly lower densities, the expandable polystyrene particles produced according to the present invention makes it possible to use thinner foam boards, which makes a space saving possible.
Surprisingly, the expandable styrene polymers of the present invention can be processed without any problems at all to give low-density foams. There are neither blowing agent losses nor disruptions of the cell structure of the foams, although a person skilled in the art would have to assume that the graphite would act as nucleating agent and lead to undesirably fine cells in the foam and poor welding together. In addition, self-extinguishing foams which pass the burning test B2 and in most cases also B1 can be produced despite the addition of graphite particles.
Owing to the incorporation of the graphite particles in the polymer matrix, there is no rubbing-off of the graphite and thus no soiling when working with such components.
The foams of the present invention can be used for thermal insulation of buildings and parts of buildings, for thermal insulation of machines and domestic appliances and also as packaging materials.